Because polylactic acid is biodegradable, many attempts have been made to develop applications of polylactic acid as a new-generation plastic. Products made from polylactic acid can be decomposed by microorganisms that are present in natural environments, in composts, etc. Therefore, polylactic acid products have an advantage in that they can be directly subjected to a biological treatment even when in admixture with easily degradable organic materials.
However, it has been found that while polylactic acid is prone to decompose in an aerobic atmosphere, polylactic acid is less susceptible to decomposition in an anaerobic atmosphere (see Patent Literature 1). Accordingly, if polylactic acid is subjected as is to methane fermentation, which requires an anaerobic atmosphere, the fermentation treatment has the disadvantage of taking a long time.
In view of the above, a method for solubilizing polylactic acid has been proposed wherein before subjecting the polylactic acid to methane fermentation, it is mixed with wastewater resulting from methane fermentation to thereby solubilize the polylactic acid at a temperature of about 50° C. to about 60° C. (see Patent Literature 2). However, when using this method, the final lactic acid yield is low, and polylactic acid cannot be efficiently solubilized; therefore, improved polylactic acid treatment efficiency and efficient energy recovery cannot be achieved by this method.
If lactic acid can be recovered after treating polylactic acid, recycling of resources can be achieved by providing a starting material for production of polylactic acid, which also contributes to global environmental protection and energy conservation. Accordingly, establishment of a technique for recovering lactic acid from polylactic acid has also been strongly desired by industry.
However, the above-mentioned polylactic acid solubilization method has the following problem. When using this method, polylactic acid is broken down into a lower molecular compound but partly remains as polymers or oligomers; therefore, lactic acid cannot be obtained with high recovery efficiency.
With such a background of prior art, there has been a desire for the development of a lactic acid production technique comprising efficiently decomposing polylactic acid, as well as an efficient energy recovery technique comprising subjecting polylactic acid to a decomposition treatment.